This invention relates to the forming of sintered material of ultra high molecular weight polyethylene (hereinafter abbreviated to "UHMW-PE") and, more particularly, to a method of producing a sintered material of UHMW-PE having an extremely high dimensional accuracy, i.e. a very small displacement (the difference between the maximum diameter and the minimum diameter) at longitudinally spaced positions such as in a rod or a pipe. Where the product has a rounded surface, the invention provides a highly accurate roundness thereof. Still further, the invention is concerned with providing a product having a highly uniform electric conductivity by ram extruding a mixture of the UHMW-PE with a lubricant, a cross-linking agent and/or an electroconductive agent.
The formed product has improved wear resistance, low temperature properties, impact resistance, stress cracking resistance, etc., as compared with the ordinary polyethylene. Round rods and pipes produced by the invention are advantageously adapted for use as bearings, gears, rollers, guide rails, timing screws, and other products requiring self-lubricating properties, a low friction coefficient and/or wear resistance.
UHMW-PE has been heretofore extruded into rods and pipe products. However, these products have had poor displacement characteristics, i.e. high displacement. For example, round rods or pipes extruded from a conventional ram extruder and fed to a cooling stand (such as round rods or pipes of UHMW-PE having larger than approximately 50 mm outer diameter) are irregularly cooled, and have been found to have variations in the dimensions of the outer diameter (i.e. displacement) of over approximately 2 mm. The external appearance of such products is deteriorated, thereby further decreasing the value of a product.
To improve the roundness of such rods or pipes, a method of producing such round rods or pipes has been attempted to limit the displacement (such as approximately 1 to 2 mm). This method includes the steps of covering the entire peripheral surface of the rod or pipe immediately upon extrusion from the extrusion outlet with a paper tube, and then uniformly gradually cooling the surface of the covered sintered material. This method has the serious disadvantage of substantially reducing the cooling rate of the product, requiring that the cooling stand by greatly increased in length, such as to 10 to 30 m.
Further, such extruded conventional UHMW-PE has been found to have greater than 10.sup.18 .OMEGA..cm volumetric intrinsic resistance, and greater than 10.sup.13 .OMEGA. surface resistance, i.e. large electric insulation. Thus, static electricity is readily charged on the surface of the extruded product, with the result that dust and particulate contaminants readily adhere to the surface of the product, presenting serious problems in many applications. Thus, the use of such a product in handling foodstuffs and the like is seriously limited. Further, such UHMW-PE products cannot be employed in atmospheres of inflammable powder dust, such as coal or coke dust capable of being exploded by a spark of a static electricity or a flame, or in an atmosphere using an organic solvent.
Resultingly, it is desirable to impart an increased electric conductivity to the UHMW-PE product. Thus, particulate carbon black, carbon fibers, or metallic powder are generally employed as an electroconductive agent to impart increased electric conductivity to the formed UHMW-PE product. However, if the ratio of the electroconductive agent to the UHMW-PE is more than 1 wt percent, the extrusion pressure increases greatly, thereby causing substantial difficulty in ram extrusion thereof to form a sintered material. If a lubricant is added to the mixture to decrease the extrusion pressure, the conductivity of the extruded product surface portion is undesirably reduced.